Preactivation

At atmospheric pressure, the conversion to trichlorosilane is limited to about 16%. The conversion of SiCl to HSiCl was found to be at equiUbrium. If contact time was greater than 45 s and the mole ratio of hydrogen to siUcon tetrachloride 1 1, then at 14 kPa (2 psi) and 550°C, the HSiCl mole fraction reached 0.7 but substantial formation of H2SiCl2 occurred (62). Enhancements in yield have been reported through preactivating the siUcon mass by removal of oxides (73) and the rapid thermal quench of the effluent gas stream (74). The reduction of siUcon tetrachloride in a plasma has also been reported (75). 

The cycle lives of several AB2 electrodes are illustrated in Fig. 20 [56], In some cases alloys require many charge-discharge cycles to become fully activated preactivation via direct reaction with H2 gas is helpful in this regard. Some pertinent properties and results are given in table 9. 

By Naylor and Gilliam62, d (pT)6 was condensed in the presence of Poly A with carbodiimide to give d-(pT)12 with a yield of 5%. In the experiment of Shabarova and Prokofiev67, d-(pA)2 preactivated in the form of an amino add amidate was condensed on Poly U to give products with a yield of 10%. Uesugi and Ts o68 studied the condensation of (2 MeIp)6 or (2 MeIp)s in the presence of a Poly C template. The relative overall yield of the oligomer products was 43 to 71%. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Titanium powder (0.626 g, 13.07 mmol) is suspended in 10 mb DME. Me3SiCl 14 (1.59 mb, 13.07 mmol) and (iPrO)3TiCl (9 mol%) are added and the mixture heated under reflux for 68 h under argon. 2-AcetylbenzaniHde 2091 (ca. 4 mmol) is added to this preactivated titanium reagent and the mixture is heated under reflux for 2.5 h. After filtration and evaporation of the filtrate the residue is chromatographed with hexane-ethylacetate (20 1) on silica gel to give 3-methyl-2-phenyl-indole 2092 as colorless crystals, m.p. 91-93 °C, in 97% yield [64] (Scheme 13.31). 

The Pd-Sn/C catalysts (1 to 7.5% Pd containing 0 to 1% Sn) were heated under vacuum at 150°C and then exposed to hydrogen. These preactivated samples were then titrated with carbon monoxide, a veiy specific ligand for Pd, up to 800 Torr at 30°C. A general linear trend of carbon monoxide concentration with % Pd in Figure 15.3 indicates that the carbon monoxide adsorption is directly correlated to Pd concentration, as expected. The trend is independent of Sn content. This linear Pd-CO trend indicates that the particle size distribution is similar for the different catalysts. However, Figure 15.3 also indicates no relationship between % H2S irreversibly adsorbed and % Pd. 

Figure 2.23. IR spectra obtained after subsequent 10 (rmol pulse adsorption of NO on a thermally preactivated Cu ZSM-S sample recorded at (a) 303 K and (b) 423 K (after [75]).

Presently the effective role of sulphur additive is not well explained because sometimes activation or deactivation phenomena are observed. Such a versatile behaviour is well-illustrated over noble metal-based catalysts particularly when they are dispersed on perovskite supports [111]. The catalytic performances of a prereduced Pt/LaCo03 in H2 at 450°C are illustrated in Figure 10.13a. After preactivation in H2 subsequent bulk and surface characterisation highlighted an extensive reduction of the perovskite... 

Pervaiz S, Hirpara JL, Clement M-V (1998) Caspase proteases mediate apoptosis induced by anticancer preactivated MC540 in human tumor lines. Cancer Lett 128 11-22... 

It should be pointed out for comparison that conventional chromatography was used to immobilize PatA onto preactivated sepharose, presumably via the only primary amino group on PatA (Bordeleau, 2005). 

Dudley and co-workers have designed pyridinium triflate reagent 58 for the preparation of benzyl ethers <06JOC3923>. This reagent is easily prepared in two steps with good yield. Since it is also bench-stable and preactivated, the benzylation reaction occurs upon warming 58 in the presence of an alcohol with no need for acidic or basic promoters. 

The gel filtration column described in step 3 should be prepared and equilibrated prior to starting the modification reaction. Enzymes preactivated with sulfo-SMCC are available from Thermo Fisher. 

A variation of the above method can be used, wherein the enzyme is first activated with SMCC and conjugated to a thiolated (strept)avidin molecule. This approach probably is the most common way of preparing (strept)avidin-enzyme conjugates, and since the preactivated enzymes are readily available (Thermo Fisher), it also may be the easiest. 

The following protocol describes the activation of HRP with sulfo-SMCC. Other enzymes may be activated in a similar manner. The activated enzyme possesses maleimide groups that are relatively unstable in aqueous solution. Therefore, the thiolation reaction should be coordinated with the activation process so that the final conjugation can be done immediately. Note If preactivated enzymes are obtained (Thermo Fisher), this step may be eliminated. 

The carboxymethyl-dextran derivative may be used to couple amine-containing molecules by the carbodiimide reaction. Heindel et al. (1994) prepared the lactone derivative of carboxymethyl-dextran by refluxing for 5 hours in toluene or other anhydrous solvents. The lactone derivative is highly reactive toward amine-containing molecules, thus creating a preactivated polymer for conjugation purposes. 

The catalyst 4b/borate was tested under similar conditions as in the case of 4a (toluene solution and liquid propylene) in propylene polymerization experiments after preactivation with TIBA (Table 1). According to the data from Table 1, the catalytic properties of 4b are inferior to those of 4a. The behavior of 4b is similar to that of asymmetric catalysts with a forward orientation of the 4-substituted indene unit [10]. The effect of the substitution position is remarkable. While the 5,7-substituted hafnocene 4a shows higher activities (up to 3.2 x 105 kg PP mol 1 Hf h 1 at 40 °C) with increasing temperatures, substantially lower or almost no activities were found for the 4,6-substituted hafnocene 4b at the same temperature (Fig. 13). 

A fiberoptic biosensor has been used for the determination of xanthine and hypoxanthine by immobilization of xanthine oxidase and peroxidase on different preactivated membranes, which were mounted onto the tip of the fiberoptic bundle [47], The hydrogen peroxide generated was measured using the luminol reaction. A linear calibration curve of the sensors occurred in the range of 1-316 nM hypoxanthine and of 3.1-316 nM xanthine, respectively, with a detection limit of 0.55 nM. 

Some porphyry copper ores contain naturally floatable gangue minerals, such as chlorites and aluminosilicates, as well as preactivated quartz. Sodium silicate, carboxy methyl-cellulose and dextrins are common depressants used to control gangue flotation. 

Dry dichloromethane was stored on preactivated molecular sieves 3 A in pellets (4 A sieves should not to be used). 

Liquid allylic alcohols ((E)-2-methyl-3-phenyl-2-propenol and (E)-2-hexen-l-ol) were stored over preactivated 3 A molecular sieves. 

Dichloromethane stored over preactivated 3 A molecular sieves, 43 mL... 

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